1. Field of the invention
The present invention relates to DNA sequences encoding an E6 or E7 fusion protein of HPV, wherein said DNA sequences are characterized by a combination of the following features: original codons are exchanged by codons which lead to an enhanced translation in a mammalian cell, they contain a deletion resulting in the production of a truncated non-functional protein, and they encode a fusion partner which is a highly immunogenic polypeptide capable of enhancing the immunogenicity of the E6- or E7-protein in the mammalian host. Furthermore, this invention relates to the modified E6- or E7-protein encoded by said DNA sequences as well as expression vectors containing said DNA sequences. Finally, the present invention relates to several uses of the above compounds, particularly as effective vaccines useful in treatment or prevention of an HPV infection or a neoplasm associated with HPV infection.
2. Description of Related Art
Carcinoma of the uterine cervix (cervical cancer, CC) is the second most common cancer in women worldwide and the first in developing countries. CC develops through premalignant intermediate stages of increasing severity known as cervical intraepithelial neoplasia (CIN) grades 1-3, the latter leading to the development of invasive cancer in about 50% of cases over a period of 1-2 decades. More than 11% of the global cancer incidence in women is due to human papillomavirus (HPV) infections. Infection with HPV types 16 and 18 has been associated with the development of CIN and CC, with HPV genotype 16 being the most prevalent viral type to infect the cervix. The E6 and E7 proteins encoded by these HPV types are thought to be involved in the pathogenesis of CC by inducing abnormal cell proliferation. Expression of E6 and E7 is consistently detected in tissue and tumor cells from HPV-associated CCs. Furthermore, the E6 and E7 genes from HPV types 16 and 18 are sufficient for transformation of epithelial cells in culture (zur Hausen, Biochim. Biophys. Acta 1288(2) (1996): F55-78).
There is increasing evidence that the E6 and E7 proteins encoded by HPV types 16 and 18 may be effective immunological targets for tumor rejection by the host. Efforts are being made to develop effective preventive and therapeutic vaccines which may be useful in treatment and prevention of a neoplasm associated to HPV. The different strategies employed so far for inducing an immune responses to proteins of the HPV types 16 and 18 are: (a) Use of synthetic antigenic peptides, (b) Use of recombinant microorganisms (recombinant bacille Calmette-Guerin; BCG), (c) use of DNA vaccines using wild-type viral genes and (d) use of Virus-like particles (VLPs).
However, unfortunately, the above strategies exhibit a variety of disadvantages which so far have hampered the development of a safe and efficient vaccine. As regards the use of synthetic antigenic peptides it has to be stressed that the identification of HPV specific, immunoreactive peptides is very complex. It requires large numbers and quantities of peptides for vaccines to be effective and of a broad spectrum. Moreover, synthetic peptides do not contain posttranslational modifications (e.g., glycosylation, sulfation, phosphorylation) normally found in native proteins and therefore are not efficient enough as vaccines. The BCG based vaccine delivery systems expressing the L1 late protein of HPV 6b or the E7 early protein of HPV 16 have been used as immunogens. However, the immune responses obtained with these systems was even less than those elicited by protein/adjuvant vaccines and, thus, this system is considered unlikely to be useful as a single component vaccine strategy. As regards DNA vaccines it has been observed that the expression of wild-type HPV genes is quite low, even if they are expressed from strong promoters, such as that of the cytomegalovirus (CMV). As regards the use of Virus-like particles (VLPs) it has to be mentioned that true VLPs are made of the L1 (capsid) protein of a specific HPV type. Therefore, they may be only useful as prophylactic rather than as therapeutic vaccines, if ever. Pseudotyped VLPs containing, for instance, epitopes of HPV-16 E7 have also been described and may be useful as prophylactic and therapeutic vaccines. However, an important limitation is that VLPs are produced in insect cells or in yeast. So far, no suitable production systems in mammalian cells have been established. Therefore critical epitopes depending on posttranslational modifications which take place in human cells are lost in these systems.